This invention relates to drive circuits for producing signals indicative of load condition. Such circuits find use in, for example, automotive vehicles, where it is desirable to monitor the condition of vehicle lamps so as to be able to warn the driver if the lamp is not operating.
Known circuits for monitoring the condition of vehicle lamps require a series resistance of known value to be inserted into the vehicle wiring harness, and use the voltage developed across the resistance to indicate the condition of a lamp supplied with current through the resistance.
Such a known circuit requires an additional component of accurately known value and produces a reliably indicative condition signal only when the load current to the circuit has stabilized after turn-on. Also, the value of the additional series resistance must be kept low in order to minimize resistive heating and to deliver sufficient energy to the lamp, but a low value resistance value produces a low voltage thereacross which results in a low signal-to-noise ratio.